A network resource such as a web page or other network-accessible content item may be associated with a number of additional resources, such as images or videos to be displayed or scripts to be executed. Software on a client computing device, such as a browser software application, typically processes embedded resource identifiers to generate requests for the content. In many cases, two or more distinct hosts or other content providers will provide client computing devices with resources associated with a single web page.
Resources embedded within or referenced by the web page may be harmful to the client computing device. For example, a web page may contain executable code that alters the state of the client computing device or collects sensitive information about the user. The executable code may have originated from a separate content source, unrelated to the provider of the web page, such as an advertising network. Existing services that warn users of potential risks associated with a requested web page can help prevent receipt of such harmful resources, but also frequently block the remainder of the web page, which may be safe. A user that deems some portion of the requested content to be important may choose to ignore such warnings in order to access the web page, thereby receiving the potentially harmful resource.